LSD-25 (lysergic acid diethylamide) is the prototypical hallucinogenic drug, whose use has seen a recent upsurge during the last several years. It also is the hallucinogen used more than any other in clinical research studies from the early 1950s to mid-1970s. Many basic discoveries regarding serotonin (5-HT) mechanisms have been made since 1980 using LSD, while human psychopharmacological validation of these animal models has been impossible because of legal restrictions on the use of LSD in humans.